As the drying rate of air-drying systems such as alkyd paints is too slow for commercial applications, it is common practice to accelerate the drying process by adding metal driers to the system. Without driers, a typical alkyd paint would take as long as days, if not weeks to dry, which is clearly undesirable for most applications.
Primary driers catalyze the formation and/or decomposition of peroxides, which are formed by the reaction of oxygen with the air-drying binder or drying oil. Cobalt carboxylates are hitherto the principal constituents, at least if drying has to take place at room temperature and within a reasonable time. The use of cobalt carboxylates, and in particular of cobalt octoates have indeed been widely described, and is common practice throughout the paint industry (e.g. J. H. Bieleman, in Additives for Coatings, ED. J. H. Bieleman, Wiley VCH, Weinheim, 2000, p. 202).
Nevertheless cobalt has shown carcinogenic effects on in vivo inhalation tests. It is generally assumed that this toxicity is related to the cobalt ion, as the tested compounds had relatively high water solubility and generated appreciable cobalt ion concentrations. The available data for most of the standard cobalt carboxylates is such that serious concern about their carcinogenicity is justified, which makes their future use as driers in auto-oxidative paint and ink systems problematic.
Whereas the cobalt carboxylate is a primary drier, other transition metals such as manganese also fulfill a role in this process. The effect of manganese carboxylates is most noticeable at higher temperatures, or else at room temperature when used as an auxiliary drier with cobalt. The higher temperatures needed for the development of the catalytic activity of manganese as primary drier are around 80° C., conditions normally found on printing presses, and hence the use of manganese driers in these applications.
GB-A-1232194 teaches the preparation and use of acidic metal bearing polymers in anticorrosive paints. The envisaged products have free carboxyl groups terminating the structure, which are assumed to be a key in forming an adhering protective film on steel. A possible activity as drier is not mentioned, with classical driers such as cobalt naphthenate being indeed added to the paint.
GB-A-550441 describes water-insoluble metal salts of polymerized rosins. This document teaches polymer salts to counter the disadvantages of existing water-insoluble non-polymerized rosins, namely their pulverulence, tendency to coalesce, and their low melting point. The reduced aqueous solubility of polymerized rosins compared to non-polymerized rosins is not taught. Moreover, the described products are not particularly alkyd-compatible as they do not contain ester functions in their structure.
Although manganese is also an essential component of life, e.g. as the central atom in SOD's (Super Oxide Dismutase), there is a known toxicology on manganese compounds. Manganese carboxylates have not been classified as yet, but it has been demonstrated that manganese carboxylates release manganese ions in aqueous solutions. Concern about the future classification of manganese carboxylates is therefore justified.
It is known that the application of printing inks on fast running rotary printing presses causes the formation of an airborne aerosol of fine ink droplets around the printing press. As the primary risk to workers is therefore absorption through inhalation, it is important to lower the water solubility, and hence the release of manganese ions at the pH values typically found in lung fluids, which is around neutral.
The present disclosure provides a new class of cobalt-bearing and manganese-bearing compounds, which retain the catalytic effects of cobalt and manganese towards the oxidative drying of polymers, while greatly avoiding toxic effects by reducing the availability of the cobalt and manganese ions in aqueous systems.
It has been shown in previous patent publications no. US20120041133 A1 and EP2370534 A1 by the inventor of the present application that it was possible to reduce the water solubility, and additionally the resulting cobalt ion concentration by including the cobalt atom into a polymeric structure. The increased molecular weight with the more complex molecular structure reduced the hydrolysis of the compounds so that the threshold values for toxicity were not attained. It has been also disclosed that this was possible only with a claimed lowest molecular weight of 3000. Furthermore it was claimed that the compounds were preferably free of phosphorus, nitrogen or sulfur atoms.
The present disclosure provides for principal use of urethane structures, so that the desired properties can be obtained at a lower average molecular weight, resulting in a lower viscosity and better solubility.